Electrochemical cell

ABSTRACT

A compact electrochemical cell utilizing a circulating electrolyte in which the anode and cathode are very closely spaced, decreasing the internal resistance of the cell, is described. The cell comprises in spaced relation a reactant chamber, a gas (bubble) distributor, an electrolyte chamber, a first electrode which is flooded with electrolyte, an electrolyte chamber, a second electrode, and a second reactant chamber. The advantages of a circulating electrolyte are achieved while permitting the use of electrodes and electrode separators having low or zero bubble pressures and while maintaining the internal resistance of the cell low.

Elnited States Patent Katz et al. Oct. 30, 1973 ELECTROCHEMICAL CELL[75] Inventors: Murray Katz, Newington; James K. f' Cums StedmanGlastonbury, both of Assistant ExammerH. A. Feeley Conn. Attorney-AlfredW. Breiner [73] Assignee: United Aircraft Corporation, [57] ABSTRACTHartford Conn A compact electrochemical cell utilizing a circulating[22] Filed; 18, 197 electrolyte in which the anode and cathode are veryclosely spaced, decreasing the internal resistance of [21] Appl 172,654the cell, is described. The cell comprises in spaced relation a reactantchamber, a gas (bubble) distributor, [52] US. Cl 136/86 R electrolytechamber, a first electrode which is 51 Int. Cl. H0lm 27/00 flooded withelectrolyte, an electrolyte chamber, a 5 Field f Search 3 3 B 3 C, 86 Rsecond electrode, and a second reactant chamber. The advantages of acirculating electrolyte are achieved 5 References Cited while permittingthe use of electrodes and electrode UNITED STATES PATENTS separatorshaving low or zero bubble pressures and 3 338 746 8/1967 Pl St t l 6186Cwhile maintaining the internal resistance of the cell u e a. 3,425,8732/1969 Worsham et al... 136/86 B 3,227,585 1/1966 Langford et al 136/86E 16 Claims, 3 Drawing Figures ELECTROCHEMICAL CELL FIELD OF INVENTIONAND BACKGROUND This invention relates to electrochemical cells and, moreparticularly, to an improvement in electrochemical cells utilizing acirculating electrolyte and achieving the advantages thereof, whilemaintaining a minute electrolyte spacing lowering the IR of the cell andpermitting use of electrodes and electrode separators having low or zerobubble pressures. For convenience, hereinafter the invention will bedescribed with reference to a fuel cell for the direct generation ofelectricity utilizing two non-consumable electrodes. As will beapparent, however, similar considerations governing the use of theinvention in fuel cells will apply to other electrochemical devicesenabling the use of the invention in such devices.

A fuel cell, as the term is employed herein, designates anelectrochemical cell for the direct generation of electrical energy froma fuel and oxidant. With such cells it is not necessary to go throughthe usual conversion of chemical energy to heat energy to mechanicalenergy to electrical energy as is common with heat engines. Such cellsin their most simplified design comprise a housing, an oxidantelectrode, a fuel electrode, and an electrolyte. In operation, it isnecessary that the fuel and oxidant contact a surface of theirrespective electrodes where a process of adsorption and desorptionoccurs leaving the electrodes electrically charged, with the secondsurface of the electrodes being in contact with the electrolyte.Depending upon the nature of the electrolyte, ions are transferredthrough the electrolyte from the anode to the cathode, or from thecathode to the anode. Electrical current is withdrawn from the cell andpassed through a suitable load where work is accomplished.

The electrolyte of fuel cells can be a solid, a molten paste,circulating free-flowing electrolyte, or a liquid trapped in ahydrophilic matrix. Although certain design considerations such ascompactness, low IR drop across the cell, and the use ofnon-self-supporting lightweight electrodes recommends cells utilizing aliquid electrolyte trapped in a hydrophilic matrix for manyapplications, there are distinct advantages in circulating electrolytefuel cell systems over matrix-type electrolyte systems. A primaryadvantage is the operational flexibility offered by the bulk electrolytein that it serves as an infinite reservoir to accommodate the volumetolerance requirements. Additionally, the circulating electrolyte servesas a convenient, heat-removable transfer fluid with no other coolantloops being required. Moreover, such fuel cells can operate onunscrubbed air by having a regenerator unit located external to the cellstack to receive the circulating electrolyte stream and remove carbondioxide and other impurities from the electrolyte before returning it tothe cell. However, circulating electrolyte fuel cells require arelatively large electrode spacing to ensure free circulation ofelectrolyte, to minimize pressure drop, and to avoid shorting of thecell due to unintentional contact between the anode and cathode of thecell. As apparent, therefore, a cell configuration offering theadvantageous features of both a circulating electrolyte cell and anelectrolyte matrix cell is imminently desirable.

OBJECTS OF THE INVENTION AND GENERAL DESCRIPTION Accordingly, a primaryobject of the present invention is to provide a fuel cell which has theadvantageous characteristics of a circulating fuel cell system whilepermitting a close spacing of the electrodes.

lt'is another object of this invention to provide a fuel cell employinga free-flowing or circulating electrolyte which minimizes internalelectrolyte polarization and simplifies the bubble pressure requirementsof the electrodes and electrode separator.

It is still another object of the present invention to provide a fuelcell employing a free-flowing or circulating electrolyte which permitsuse of electrodes and electrode separators with low or zero bubblepressures.

It is another object of this invention to provide a fuel cell employinga free-flowing or circulating electrolyte with its inherent infiniteelectrolyte reservoir to accommodate the volume tolerance requirementsof the cell while having the advantages of a matrix-type cell.

It is another object of this invention to provide a fuel cell which usesgaseous reactants and a liquid circulating electrolyte and obtainsmaximum utilization of the reactants.

It is another object of this invention to provide a fuel cell utilizinga gas distributor in functional association with a cell electrodewhereby a gaseous reactant intermixed with a flowing electrolytecontacts said electrode over substantially its entire reactive face.

These and other objects of the invention will be more readily apparentfrom the following detailed description, with particular emphasis beingplaced on the embodiments illustrated in the drawing.

In accordance with the present invention, a circulating electrolyte fuelcell is constructed which comprises in spaced relation a reactantchamber, a gas (bubble) distributor, an electrolyte chamber, a firstelectrode which is flooded with electrolyte, an electrolyte chamber, asecond electrode, and means for feeding a second reactant to said secondelectrode. In accordance with this construction, the anode and cathodeto the electrochemical cell are preferrably separated by a thinnon-electrically conductive and ion-conductive separator which is orbecomes impregnated with electrolyte. Alternatively, it is possible toconstruct the cell merely using an electrolyte chamber or space betweenthe electrodes. In operation, the space will fill with electrolyte.However, in the latter construction care must be taken to avoid shortingof the cell as a result of the electrodes touching or contacting eachother. The electrolyte is circulated in the electrolyte chamber adjacentthe first electrode essentially flooding the electrode, feeding into theelectrolyte chamber between the electrodes and interchanging with theelectrolyte in the chamber. The reactants are fed to the cell underpressure. The reactant gas or bubble distributor, i.e., a sintered ormultiple orifice plate, causes the reactant gas is not required toprovide the primary advantages of thepresent system The presentconstruction permits a narrow spacing between the electrodes incomparison to designs of prior art circulating electrolyte systems wherethe electrolyte is circulated between electrodes requiring largespacings to minimize pressure drop and/or to permit built-in barrierswith large bubble pressures. As a result of the thin electrode spacings,electrolyte ohmic polarizations are minimized resulting in higher powerdensities. Moreover, the present construction permits the use of thinelectrodes and electrode separators with low or zero bubble pressuresince the gas pressure in the bubble is virtually equal to theelectrolyte pressure at the electrode. More specifically, in the designswhere electrolyte is only circulated behind one electrode, if theunflooded electrode has a high bubble pressure, the need for a highbubble pressure matrix or high bubble pressure counterelectrode iseliminated. If a single high bubble pressure matrix is used, then lowbubble pressure electrodes may be used for the anode and cathode similarto the anodes and cathodes employed in trapped electrolyte cells.Further, in the event the electrolyte is caused to flow by the face ofboth electrodes with each reactant bubbled into the respective streams,the need for a high bubble pressure matrix separator, and high bubblepressure fuel and oxidant electrodes is completely eliminated. Thepresent design, in addition to the aforesaid advantages, has theinherent advantages of a free-flowing electrolyte system which includesheat removal, water removal, control of humidity of the cell, and theability to use fuels or oxidants which contaminate the electrolyte suchas the hydrocarbons or carbon oxides with a basic electrolyte, with theelectrolyte being purified in an external unit.

THE DRAWING AND SPECIFIC EMBODIMENT In order to more specificallyillustrate the invention, reference is made to the drawing wherein FIG.1 is a transverse sectional view of a single cell constructed inaccordance with the present invention where the electrolyte iscirculated behind only one of the electrodes of the cell;

FIG. 2 is a frontal view of one electrode of the cell; and

FIG. 3 is a transverse sectional view of a single cell illustrating asecond embodiment of the invention where the electrolyte is circulatedbehind both of the electrodes of the cell.

In the drawing, like numerals designate like parts throughout. 1

Referring to the drawing, fuel cell 10 comprises anode 5 and cathode 7spaced apart by a separator 6. In the embodiment shown, electrodes 5 and7 are lightweight screen electrodes comprising a conductive nickelscreen embedded in a uniform admixture of catalytic metal, in thisinstance platinum, and polytetrafluoroethylene particles. The ratio ofplatinum to polytetrafluoroethylene on a volume basis is 3 to '7, withthe platinum loading of the electrode being 10 mglcm The electrodes areapproximately 5 mils in thickness. The electrode separator 6 is pressedasbestos and is approximately 5 mils in thickness. The porous nickelsinter 20, approximately mils in thickness, is spaced adjacent to anode5 with the anode and sinter forming a chamber 22 there between anode 5and housing 28 forming a second chamber 24. A further chamber 26is'formed adjacent cathode 7 by the cathode and housing 28. The entirecell assembly is held together with threaded tie rod 38 at either end ofthe cell. It is to be understood that although in the embodiment shownin FIG. 1, the anode is the flooded electrode, it can just as well bethe cathode.

In operation, a 30-percent aqueous potassium hydroxide electrolytesolution is pumped into electrolyte chamber 22 through inlet 22a at acontrolled rate where it floods anode 5 and is removed from the cellthrough exit 22b. The electrolyte, after it floods the anode, will floodseparator 6. Due to mixing, electrolyte within electrode separator 6 iscontinually exchanged with the electrolyte in chamber 22 and alsomaintaining the electrolyte volume within the separator constant. Areactant gas, in this instance hydrogen, is fed to anode chamber 24through inlet 240 with excess gas being removed through outlet 24b. As aresult of the gas being under pressure, the gas will pass through bubbledistributor 20 causing bubbles to eject into the electrolyte directly atthe face of electrode 5. The pressure and plate design are such toensure that the bubble flow through the electrolyte is at a high volumein comparison to the velocity of the electrolyte stream to maximize theamount of gasreaching the electrode and to minimize diffusionpolarization at the electrode. FIG. 2 shows a frontal view of electrode5 with the major portion of the electrode exposed to reactant bubblesand the flow of turbulent electrolyte between the bubbles. An oxidant,in this instance air, is fed into reactant chamber 26 through inlet 26acontacting cathode 7, with excess air and impurities being ventedthrough exit 26b. The cell when operated at a constant current drainwill provide a substantially constant cell output. There is littlefluctuation in current characteristics of the cell since the entirevolume tolerance function in the cell is controlled by the circulatingelectrolyte. Moreover, as noted hereinbefore, the bubble pressure of theanode S and separator 6 can be very low, only requiring a high bubblepressure at cathode 7. Alternatively, separator 6 can have a high bubblepressure, permitting the use of a cathode having a low bubble pressure.

FIG. 3 illustrates the cell substantially similar to that shown inFIG. 1. However, in this instance a second electrolyte chamber ismaintained behind cathode 7 in combination with a second bubbledistributor 20. Utilizing this design, a more uniform control of thethermal characteristics of the cell is realized. Moreover, thisembodiment completely eliminates the need for a high bubble pressurematrix separator and/or high bubble pressure fuel and oxidantelectrodes.

Although the present invention has been described with reference tolightweight electrodes comprising a metal support screen embedded in acatalytic mixture of metal and hydrophobic plastic binder, otherelectrodes, such as porous metal sinters, carbon disc, and the like canbe employed. Moreover, although it is indicated that the electrodeseparators are made of asbestos, other hydrophilic separators can beutilized including ceramic materials and polymeric materials. Inaddition to porous nickel sinters, the bubble distributor can be made ofany material which is resistant to the corrosive influences of theelectrolyte including porous copper, tantalum, iron, and the like.Moreover, it is not necessary that the bubble distributor be a metalsinter. A multiple orifice plate constructed of metal or a plastic canbe utilized with it only being essential that the distributor directreactant gas directly to the face of the electrode through the turbulentflowing electrolyte.

The operating temperature of the cell can vary as long as it is notabove the critical temperature of the electrodes and/or electrodeseparator being utilized. Preferably, the operating temperature of thecell will be in the range of from about to 250C. In addition to thepotassium hydroxide electrolyte disclosed hereinbefore, other commonlyemployed aqueous electrolytes exemplified by aqueous solutions of thealkali hydroxides, alkaline earth hydroxides, and carbonates; as well asstrong acid electrolytes such as hydrochloric acid, sulfuric acid,nitric acid, and phosphoric acid can be used. Commonly employedreactants in addition to hydrogen and oxygen can be utilized in thecells of the present invention.

Although the present invention is described and illustrated in thedrawing with reference to single cells, it will be apparent that in thepreferred construction a plurality of cells will be stacked togetherutilizing manifold feed arrangements for the fuel and oxidant as well asa manifold system for circulating the electrolyte through the pluralityof cells. In constructing the cell stack, it may be desirable to arrangethe cells in order that a single reactant chamber will service theelectrodes of adjacent cells. This will contribute to the compactness ofthe cell stack. As will be apparent to those skilled in the art, variousother modifications can be made in the over-all cell design to meetoperating conditions. For example, a stack of cells employing theconcept of this invention can utilize a regenerator system with thecirculating electrolyte in order to remove carbon dioxide and/or otherimpurities from the electrolyte. A regenerator unit in combination witha fuel cell is described, for example, in US. Pat. No. 3,331,703. Thesemodifications being within the ability of one skilled in the art are tobe covered herein with the invention only being limited in accordancewith the appended claims.

It is claimed:

1. A fuel cell having a pair of opposed electrodes electricallyconnected through a work load, a first noncirculating electrolytechamber between said pair of electrodes, a porous gas distributor platepositioned behind and spaced from at least one of said pair ofelectrodes, said one electrode and distributor plate forming a secondelectrolyte chamber, said second electrolyte chamber being behind saidone of said electrodes, a reactant chamber behind and at the surface ofsaid porous gas distributor plate not fronting the said secondelectrolyte chamber; inlet, outlet, and flow control means constructedand arranged with said second electrolyte chamber for controlled flow ofelectrolyte through said chamber; at least said one electrode beinghydrophilic permitting electrolyte communication from said secondelectrolyte chamber with said first electrolyte chamber between saidpair of electrodes through said one electrode.

2. The fuel cell of claim 1 wherein a porous gas distributor plate ispositioned behind and spaced from each of said pair of electrodesthereby forming an electrolyte chamber as defined behind each of saidelectrodes.

3. The fuel cell of claim 1 wherein the said electrode is a homoporouselectrode comprising a uniform admixture of catalyst metal andhydrophobic polymer having a substantially zero bubble pressure.

4. The fuel cell of claim 2 wherein both of said electrodes is ahomoporous electrode comprising a uniform admixture of catalyst metaland hydrophobic polymer having a substantially zero bubble pressure.

5. The fuel cell of claim 1 wherein the bubble distributor is a poroussinter having a porosity of from about TO to perc ent.

6. The fuel cei of claim 5 wherein the porous sinter is nickel.

7. The fuel cell of claim I- wherein the bubble distributor is a porousplate having a plurality of orifices.

8. The fuel cell of claim 7 wherein the porous plate is metal.

9. The fuel cell of claim 7 wherein the porous plate is plastic.

10. The fuel cell of claim 2 wherein the bubble dis tributors are eachporous sinters having a porosity of from about 10 to 80 percent.

11. The fuel cell of claim 2 wherein the bubble distributors are eachporous plates having a plurality of orifices.

12. The fuel cell of claim 11 wherein the porous plate is metal.

13. The fuel cell of claim 11 wherein the porous plate is plastic.

14. The process of generating electrical energy in a fuel cellcomprising a fuel electrode, an oxidant electrode, and a firstnon-circulating electrolyte chamber between said electrodes, said cellhaving in combination therewith a porous gas distributor positionedbehind and spaced from at least one of said electrodes to provide asecond electrolyte chamber behind said one of said electrodes, includingthe steps of feeding a gas to said gas distributor at the surfacethereof not in contact with said electrolyte chamber under controlledconditions and feeding an electrolyte into said second electrolytechamber, whereby said reactant gas will pass thorugh said porousdistributor and into said free flowing electrolyte in said secondelectrolyte chamber and to the face of said one electrode, said flowingelectrolyte in said second electrolyte chamber continuouslyinterchanging with electrolyte in said first electrolyte chamber throughsaid one electrode.

15. The fuel cell of claim 7 wherein the porous plate is carbon.

16. The fuel cell of claim 1 1 wherein the porous plate is carbon.

QR t 8:

2. The fuel cell of claim 1 wherein a porous gas distributor plate ispositioned behind and spaced from each of said pair of electrodesthereby forming an electrolyte chamber as defined behind each of saidelectrodes.
 3. The fuel cell of claim 1 wherein the said electrode is ahomoporous electrode comprising a uniform admixture of catalyst metaland hydrophobic polymer having a substantially zero bubble pressure. 4.The fuel cell of claim 2 wherein both of said electrodes is a homoporouselectrode comprising a uniform admixture of catalyst metal andhydrophobic polymer having a substantially zero bubble pressure.
 5. Thefuel cell of claim 1 wherein the bubble distributor is a porous sinterhaving a porosity of from about 10 to 80 percent.
 6. The fuel cell ofclaim 5 wherein the porous sinter is nickel.
 7. The fuel cell of claim 1wherein the bubble distributor is a porous plate having a plurality oforifices.
 8. The fuel cell of claim 7 wherein the porous plate is metal.9. The fuel cell of claim 7 wherein the porous plate is plastic.
 10. Thefuel cell of claim 2 wherein the bubble distributors are each poroussinters having a porosity of from about 10 to 80 percent.
 11. The fuelcell of claim 2 wherein the bubble distributors are each porous plateshaving a plurality of orifices.
 12. The fuel cell of claim 11 whereinthe porous plate is metal.
 13. The fuel cell of claim 11 wherein theporous plate is plastic.
 14. The process of generating electrical energyin a fuel cell comprising a fuel electrode, an oxidant electrode, and afirst non-circulating electrolyte chamber between said electrodes, saidcell having in combination therewith a porous gas distributor positionedbehind and spaced from at least one of said electrodes to provide asecond electrolyte chamber behind said one of said electrodes, includingthe steps of feeding a gas to said gas distributor at the surfacethereof not in contact with said electrolyte chamber under controlledconditions and feeding an electrolyte into said second electrolytechamber, whereby said reactant gas wiLl pass thorugh said porousdistributor and into said free flowing electrolyte in said secondelectrolyte chamber and to the face of said one electrode, said flowingelectrolyte in said second electrolyte chamber continuouslyinterchanging with electrolyte in said first electrolyte chamber throughsaid one electrode.
 15. The fuel cell of claim 7 wherein the porousplate is carbon.
 16. The fuel cell of claim 11 wherein the porous plateis carbon.